Field of the Invention
The present invention relates to a first composition (A) comprising (i) at least one compound (I) selected from the group consisting of triallyl isocyanurate, triallyl cyanurate, wherein the compound (I) is preferably triallyl isocyanurate; and (ii) at least one urea compound. The present invention additionally relates to a second composition (B) comprising the first composition (A) and at least one polyolefin copolymer. The present invention finally relates to the use of the composition (B) for producing a film for encapsulating an electronic device, in particular a solar cell.
Discussion of the Background
Photovoltaic modules (photovoltaic=“PV”) typically consist of a layer of symmetrically arranged silicon cells welded into two layers of a protective film. This protective film is in turn itself stabilized by a “backsheet” on its reverse side and a “frontsheet” on its front side. The backsheet and frontsheet may either be suitable plastics material films or may be made of glass. The function of the encapsulation material is essentially to protect the PV module from weathering effects and mechanical stress, and for that reason the mechanical stability of the particular encapsulation material is an important property. In addition, good encapsulation materials exhibit a rapid curing rate, high gel content, high transmission, low tendency to temperature- and heat-induced discolouration and high adhesion (i.e. a low propensity for UV-induced delamination).
The encapsulation materials described for this purpose in the related art (for example WO 2008/036708 A2) are typically based on materials such as silicone resins, polyvinyl butyral resins, ionomers, polyolefin films or ethylene-vinyl acetate copolymers (“EVA”).
Processes for producing such encapsulation films are familiar to those skilled in the art (EP 1 164 167 A1). In these processes the crosslinkers, together with a polyolefin copolymer (and possibly further additives), are homogeneously mixed in an extruder for example, and then extruded to give a film. The process described in EP 1 164 167 A1 relates to encapsulation films based on EVA but is also applicable to films made of other materials, for example those mentioned hereinabove.
The encapsulation of the silicon cells is typically performed in a vacuum lamination oven (EP 2 457 728 A1). To this end, the layer structure of the PV module is prepared and initially heated up gradually in a lamination oven (consisting of two chambers separated by a membrane). This softens the polyolefin copolymer (for example EVA). The oven is simultaneously evacuated to remove the air between the layers. This step is the most critical and takes between 4 and 6 minutes. Subsequently, the vacuum is broken via the second chamber, and the layers of the module are welded to one another by application of pressure. Heating is simultaneously continued up to the crosslinking temperature, the crosslinking of the film then taking place in this final step.
The use of EVA in particular is standard in the production of encapsulation films for solar modules. However, EVA also has a lower specific electrical resistance ρ than polyolefins for instance. This makes the use of EVA films as encapsulation material less attractive, since it is specifically encapsulation materials having a high specific electrical resistance ρ that are desired.
This is because the so-called “PID” effect (PID=potential-induced degradation) currently represents a major quality problem for PV modules. The term “PID” is understood to mean a voltage-induced performance degradation caused by so-called “stray currents” within the PV module.
The damaging stray currents are caused not only by the structure of the solar cell but also by the voltage level of the individual PV modules with respect to the earth potential—in most unearthed PV systems, the PV modules are subjected to a positive or negative voltage. PIP usually occurs at a negative voltage relative to the earth potential and is accelerated by high system voltages, high temperatures and high atmospheric humidity. As a result, sodium ions migrate out of the cover glass of the PV module to the interface of the solar cell and cause damage (“shunts”) there, which may lead to performance losses or even to the total loss of the PV module.
The risk of a PID effect occurring may be markedly reduced by increasing the specific electrical resistance ρ of the encapsulation films.
The specific electrical resistance p or else volume resistivity (also abbreviated to “VR” hereinbelow) is a temperature-dependent material constant. It is utilized to calculate the electrical resistance of a homogeneous electrical conductor. Specific electrical resistance is determined in accordance with the invention by means of ASTM-D257.
The higher the specific electrical resistance ρ of a material, the lower the susceptibility of the photovoltaic modules to the PID effect. One significant positive effect of increasing the specific electrical resistance p of encapsulation films is thus an increase in the lifetime and efficiency of PV modules.
The related art discusses the problem of the PID effect in connection with encapsulation films for PV modules in CN 103525321 A. This document describes an EVA-based film for encapsulating solar cells, which comprises triallyl isocyanurate (“TAIC”) and trimethylolpropane trimethacrylate (“TMPTMA”) as co-crosslinkers and, as further additives, preferably comprises a polyolefin ionomer and a polysiloxane for hydrophobization. This film exhibits a reduced PID effect. However this film has the disadvantage that polyolefin ionomers are relatively costly. Polysiloxanes moreover have an adverse effect on adhesion properties. In addition, the examples do not give any specific information as to the improvements achievable with particular concentrations.
JP 2007-281135 A also describes a crosslinker combination of TAIC and TMPTMA. The TMPTMA acts as an accelerant for the crosslinking reaction and thus brings about enhanced productivity.
JP 2012-067174 A and JP 2012-087260 A describe an encapsulation film for solar cells based on EVA/a polyolefin, which comprises not only TAIC but also, for example, ethylene glycol dimethacrylate, neopentyl glycol dimethacrylate, hexane-1,6-diol dimethacrylate as crosslinkers. These co-crosslinkers initially retard the crosslinking reaction somewhat and thus increase the processing time window.
JP 2009-135200 A likewise describes crosslinkers comprising TAIC and various (meth)acrylate derivatives of polyfunctional alcohols, improved heat resistance coupled with a reduced tendency for delamination of the EVA-based encapsulation being described in this case.
JP 2007-281135 A and JP 2007-305634 A describe crosslinker combinations of TAIC and trimethylolpropane triacrylate (“TMPTA”) for use in the production of multilayer co-extruded EVA encapsulation films for solar cells.
Similar combinations of crosslinkers for solar cell encapsulation films are described, for example, by JP 2013-138094 A, JPH11-20094, JPH11-20095, JPH11-20096, JPH11-20097, JPH11-20098, JPH11-21541, CN 102391568 A, CN 102504715 A, CN 102863918 A, CN 102911612 A, CN 103045105 A, CN 103755876 A, CN 103804774 A, US 2011/0160383 A1, WO 2014/129573 A1.
There is accordingly a need for novel co-crosslinker systems, in particular for producing encapsulation films for solar cells, which, compared to films crosslinked in accordance with the related art, result in a markedly increased electrical resistance and thus lead to a reduction in the PID risk when employed in photovoltaic modules.